As a typical material used as a gain medium for a solid-state laser oscillator, there is known a dielectric bulk of, for example, Y.sub.3 Al.sub.5 O.sub.12 doped with a luminous element such as lanthanide series. The form of such a bulk is in general single crystal or amorphous. This is because, if a bulk material of some other form (mass of polycrystalline or crystalline grains) is placed in an optical resonator, the resonator loss increases due to light scattering occurring inside the bulk, making it impossible to obtain a sufficient laser output.
In order to further increase the output of a solid-state laser, it is necessary to enlarge the gain medium of the laser oscillator in size to increase the gain length or to increase the percentage of a luminous element (dopant concentration) doped in the gain medium. However, using a large-sized single crystal is not advantageous in terms of cost, and present-day techniques of single crystal growth place restrictions on increasing the doping concentration of the luminous element.
If an amorphous material doped with a luminous element such as lanthanide series is used as the gain medium, then it is relatively easy to increase the size of the gain medium and to increase the doping concentration of the luminous element. Amorphous materials in general are, however, low in heat conductivity and difficulty arises in efficiently dissipating heat generated during high-output operation of the laser oscillator, possibly causing thermal breakdown of the materials. It is, therefore, difficult to use an amorphous material doped with a luminous element such as lanthanide series, as the gain medium for a high-output laser.
Thus, there is a difficulty in increasing the output of a laser oscillator by using, as its gain medium, a material doped with a luminous element such as lanthanide series and having the form of single crystal or amorphous bulk. To resolve the situation, attempts have been made to use as the gain medium of a laser oscillator a material doped with a luminous element such as lanthanide series and having the form of numerous dielectric grains.
These attempts are based on the idea that dielectric grains consisting of high-quality single crystals can be relatively easily produced at low cost, and since a large heat radiating surface area is ensured even if the dielectric grains are amorphous, the possibility of thermal breakdown lessens and increase in the output of a laser oscillator can be expected.
However, where dielectric grains are used as the gain medium, increased gain volume for higher output of a laser oscillator constitutes an additional cause of the resonator loss, impeding increase of the output of the laser oscillator. Specifically, in cases where dielectric grains are used as the gain medium, if the density of dielectric grains is increased to thereby increase the gain volume, then granular-boundary scattering of a laser beam occurring between the dielectric grains intensifies and thus the resonator loss increases, making it difficult to increase the laser oscillator output in practice.